Lucius Modestus, the celebrated bath architect of ancient Rome, has become a sensation across the empire. His innovative designs, unknowingly inspired by the bath culture of modern-day Japan, have made him a legend. He is now summoned by Emperor Hadrian for his most important commission yet: to build a magnificent, healing bath complex within the Colosseum to soothe and restore the gladiators who fight there. Struggling under the weight of this imperial order, Lucius finds himself in his familiar predicament. Plunging into the waters of a Roman bath, he is mysteriously pulled through time and space once again, resurfacing in the baths of 21st century Japan.

This time, his journey through the Land of the Flat-Faced People is more chaotic and wondrous than ever. He encounters the massive, powerful bodies and unique customs of sumo wrestlers in their communal bath, drawing inspiration for the gladiators' facilities. He stumbles upon modern marvels like stone footpaths for massage, bubbling bath powders, and the thrilling, serpentine descent of a water slide, which he adapts into a spectacular attraction for Roman children. During his adventures, he is reunited with the kind-hearted Japanese manga artist, Mami Yamakoshi, who has a unique ability to communicate with him in Latin. Together, they experience everything from the savory pleasures of ramen and gyoza to the startling efficiency of a high-tech toilet, all of which Lucios misinterprets through his Roman-centric worldview.

However, the stakes are far higher than a simple quest for architectural inspiration. Lucius inadvertently becomes entangled in a dangerous political schism threatening to tear the Roman Empire apart. The wise Emperor Hadrian desires peace and seeks to use the concept of a healing utopia, which Lucius calls a "Utopia," to unify his people. Opposing him is the powerful Senate, led by the ambitious Ceionius, who wants to expand the empire through brutal force and sees Lucius as a pawn or a threat to be eliminated. As Lucius races to complete his Utopia Baths, he is forced to confront not only the technical challenges of building but also the perils of imperial politics. He must protect his friends, including Mami who finds herself a stranger in a strange land after being caught in one of his time slips, and prevent his life's work from being destroyed by civil war.

From his initial task for the gladiators, Lucius’s mission expands into a desperate attempt to bridge a growing rift between the Emperor and the Senate, with the future of the empire hanging in the balance. Using his wits and the surprisingly adaptable innovations from his time in Japan, he strives to create a bath so magnificent, so restorative, that it might just remind a fractured Rome of the virtues of peace and community. The film culminates in a grand, often absurd, finale that pits Roman legions against sumo wrestlers and the power of a good soak against the forces of war, as Lucius discovers that the greatest bath he can build might be one that heals an entire civilization.